Born out of a drive to better protect investors in the US railways in the 19th century, credit rating agencies have been providing analysis of the financial health of companies and national governments ever since.
Large-scale borrowers are judged using a marking system which essentially gives buyers of the debt an informed idea of how likely the company or country is of paying up.
The three leading agencies are Standard & Poor's, Moody's and Fitch, but all will provide ratings after examining a range of financial and business attributes.
The "triple A" rating, which Moody's has just stripped from the UK, is the gold standard for borrowers to attain.
- Read: ITV News' Economics Editor Richard Edgar examines whether Britain's credit rating blow has downgraded the Chancellor's reputation.
It usually enables the borrower to secure loans at lower interest rates, though the rating is not a guarantee that the AAA bond will not default.
A downgrading can spell clear trouble for some borrowers, but ratings have been known to be regained if a nation's finances are strengthened.
Today's markets will take the keenest of interest in the fluctuating credit ratings of the biggest borrowers.
But they are also increasingly looking at the individual country's fundamentals themselves beyond the traditional agencies.